Field of the Invention
The invention relates to a method for adjusting a BGR (Bandgap Reference) circuit and to a BGR circuit that can be adjusted using the method.
Circuits that generate a constant output voltage that is independent of temperature and supply voltage fluctuations are required for various applications in semiconductor circuit technology. They are used in both analog, digital and hybrid analog/digital circuits. One type of such circuits often used are known as BGR circuits (bandgap reference circuits).
The basic principle of a BGR circuit consists in adding two partial signals (voltages or currents) which have a mutually opposite temperature response. While one of the two partial signals falls as the temperature increases, the other partial signal rises as the temperature increases. The output voltage that is constant with temperature over a certain range is then derived from the sum of the two partial signals. The output voltage of a BGR circuit is also referred to as a reference voltage hereinafter in accordance with the customary usage.
A known problem in the case of BGR circuits is that circuits from the same production series have different reference voltages. In practice, it is often necessary, therefore, to adjust the BGR circuit in order to obtain a sufficient accuracy with regard to the desired absolute reference voltage value and/or the desired temperature constancy of the reference voltage.
BGR circuits have both passive components, e.g. resistors, and active components, usually in the form of a differential or operational amplifier. A deviation of the reference voltage from the ideal, calculated value and from a constant temperature response is attributed to a lack of matching of the passive and active components.
The aim of adjusting a BGR circuit consists, on the one hand, in minimizing a deviation of the reference voltage value obtained at a specific temperature from a value calculated with respect to this temperature and, on the other hand, in optimizing the temperature characteristic of the reference voltage, i.e. in obtaining a flat voltage/temperature characteristic curve.
The following methods have been disclosed heretofore for adjusting BGR circuits:
In a first known method, an offset compensation is performed directly at the amplifier that generates the offset. Most operational amplifiers have suitable actuating inputs for this purpose. An offset compensation eliminates the predominant error component of the deviation between the reference voltage value obtained at the output of the circuit and the calculated value. What is disadvantageous, however, is that a residual deviation of the aforementioned parameters generally remains and that an optimum temperature characteristic of the reference voltage is not obtained, rather, on the contrary, it is often the case that the temperature characteristic is even impaired by this step.
In a second known method, the output voltage of the circuit (i.e. the reference voltage) is set directly to the calculated value by a regulable resistor or another passive component of the circuit. In this way, the correct voltage value is obtained at the temperature at which the setting is effected. What is disadvantageous is that an optimum temperature constancy of the reference voltage cannot be guaranteed in the case of this method.
BGR circuits that have to meet very stringent requirements with regard to the absolute value and the temperature constancy of the reference voltage have to be optimized both with regard to their absolute value (which is predominated by the offset error) and with regard to their temperature response. Such BGR circuits have to be adjusted at two different temperatures. The high complexity required for this is disadvantageous.
U.S Pat. No. 6,118,264 describes a BGR circuit that is connected to an adjustment device. The adjustment device generates a compensation voltage that is added to the BGR voltage provided by the BGR circuit, as a result of which a reference voltage is generated. The compensation voltage has an opposite temperature characteristic to the BGR voltage over specific temperature ranges. Overall, this results in an improved temperature characteristic of the reference voltage.
It is accordingly an object of the invention to provide a circuit for generating a temperature-stabilized reference voltage and a method for adjusting the circuit to provide a predetermined value of the reference voltage, which overcomes the above-mentioned disadvantages of the prior art apparatus and methods of this general type.
The adjustment method for BGR circuits is simple to carry out and makes it possible to achieve a good temperature constancy of the reference voltage and a good correspondence between the reference voltage value and an expected or calculated voltage value. Furthermore, the invention provides a BGR circuit that can be adjusted in a simple manner
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for adjusting a circuit for generating a temperature-stabilized reference voltage to generate a predetermined value of the reference voltage. The method includes steps of: constructing the circuit from a voltage differential amplifier and an external circuitry including at least one component with a variable resistance, the external circuitry assigned to the voltage differential amplifier; performing an offset adjustment of the voltage differential amplifier at a predetermined temperature; and subsequently adjusting the reference voltage to the predetermined value of the reference voltage at the predetermined temperature by setting the variable resistance of the component.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a circuit for generating a temperature-stabilized reference voltage. The circuit includes: a voltage differential amplifier having an inverting input, a noninverting input, and an output; a device for offset correction assigned to the voltage differential amplifier; and an external circuitry configured external to the voltage differential amplifier. The external circuitry is connected to the inverting input, the noninverting input, and the output of the voltage differential amplifier. The external circuitry is constructed such that the output voltage of the voltage differential amplifier corresponds to a sum of at least two signals each having a temperature characteristic. The temperature characteristic of one of the two signals has a sign that is different than the temperature characteristic of another one of the two signals. The external circuitry includes at least one component having a variable resistance for influencing the temperature characteristic of at least one of the two signals. The external circuitry includes a first switching device for isolating the inverting input and the noninverting input of the voltage differential amplifier from the external circuitry. The external circuitry includes a second switching device for short-circuiting the inverting input and the noninverting input of the voltage differential amplifier.
The adjustment method includes two adjustment steps that are carried out one after the other: in a first adjustment step, an offset adjustment of the voltage differential amplifier is carried out at a predetermined temperature. In a second adjustment step, the value of the reference voltage obtained during the first adjustment step is then set to the predetermined (i.e. calculated) value of the reference voltage for this circuit.
The particular advantage of the method is that the two adjustment steps are carried out at one and the same temperature and in this case, an adjustment is brought about with regard both to the absolute value and to the temperature characteristic of the reference voltage obtained.
The term xe2x80x9cvoltage differential amplifierxe2x80x9d means any type of an amplifier that is designed to amplify a voltage difference. In particular, the term encompasses a differential amplifier and an operational amplifier.
An advantageous procedure when carrying out the first adjustment step is characterized in that this step includes the substeps of short-circuiting the inputs of the voltage differential amplifier and regulating the output voltage of the voltage differential amplifier to a predetermined voltage value. The predetermined voltage value may be, in particular, the common mode voltage, which is the mean of the positive and negative potentials of the operating voltage of the voltage differential amplifier. The voltage differential amplifier is preferably operated as a comparator during the offset adjustment.
In accordance with an additional feature of the invention, the inputs of the voltage differential amplifier can be isolated from the external circuitry by a first switching device and can be short-circuited by a second switching device. In this configuration of the circuit, the short-circuit adjustment of the voltage differential amplifier can then be performed for the purpose of offset correction. Afterward, the inputs of the voltage differential amplifier can be connected to the external circuitry again by the first switching device and the short circuit of the inputs can be cancelled by the second switching device. In this configuration of the circuit, the adjustment of the output voltage of the circuit to the predetermined value of the reference voltage can then be carried out by varying the resistance of at least one component having an adjustable resistance. This adjustment has the effect that a virtually constant, i.e. temperature-independent, reference voltage is established in a certain range around the predetermined temperature.
The advantages of this BGR circuit are that the same circuit can be used both to compensate for the voltage offset of the voltage differential amplifier and to carry out the adjustment of the passive components of the circuit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for adjusting a BGR circuit and BGR circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.